1. Field of the Invention
The invention relates to a digitizing tablet, more particularly to a method and apparatus for detecting the position of a coordinate probe relative to a digitizing tablet.
2. Description of the Related Art
Generally, a digitizing tablet is used with a coordinate probe that radiates an electromagnetic coordinate signal. The coordinate signal is received by a sensing coil grid that is provided on a working region of the digitizing tablet. When the coordinate probe is located between two sensing coils of the grid, the electric currents induced in these sensing coils have a 180.degree. phase difference therebetween, as shown in FIG. 1. A series of processing circuits, such as signal amplifiers, filters, signal conditioners, counters and processors, are employed to process the electric currents induced in the sensing coils of the grid so as to generate a corresponding coordinate data output.
It is noted that the circuitry needed to process the current signals captured in the conventional digitizing tablet is relatively complicated, thereby resulting in relatively high operating power consumption and relatively high manufacturing costs. Particularly, the conventional digitizing tablet incorporates a multiplexer array associated with the sensing coil grid for connecting the latter to the processing circuitry. The multiplexer array includes a plurality of switches, and each switch is connected to one of the sensing coils. Since the number of switches is equal to the number of sensing coils in the grid, the conventional digitizing tablet is relatively costly to implement.
Further, there are two types of coordinate probes, wired and wireless, available for use with a digitizing tablet. Wired coordinate probes require a cable connection with the digitizing tablet for receiving an excitation signal therefrom. Once excited, the wired coordinate probes radiate the coordinate signal that is received by the sensing coil grid on the working region of the digitizing tablet. On the other hand, wireless coordinate probes do not require a cable connection with the digitizing tablet, and are capable of being activated to radiate the coordinate signal that is received by the sensing coil grid on the working region of the digitizing tablet. The sensing coils of the grid are scanned sequentially with the use of the multiplexer array to determine the position of the coordinate probe relative to the digitizing tablet.
However, regardless of the type of coordinate probe that is in use, erroneous operation of the digitizing tablet, which can inconvenience the user, easily occurs due to phantom images arising from noise or detection of electric current induced in the sensing coils albeit the coordinate probe is not disposed on the working region of the digitizing tablet.